1. Field of the Invention
This invention pertains generally to moisture detection in soil, and more specifically to a method and apparatus using principles of thermal conductance to detect intrusion of moisture into soil and landfills containing toxic and/or radioactive waste.
2. Description of the Related Art
Thousands of landfills exist across the U.S. which are operated by both government and private entities. Many such landfills contain chemical waste, low-level radioactive waste and/or mixed waste. Moisture intrusion into the soil comprising such landfills represents a problem in that wastes can be destabilized or even mobilized resulting in inadequate containment. Moisture, from precipitation (rain and snow, for example), or from overland runoff can mobilize waste resulting in contamination of ground water sources thereby creating a health hazard. Therefore, effective and economical monitoring of the presence and movement of moisture in landfills is often critical to environmental safety and remediation programs.
Various containment methods including using barrier covers, liners and in-situ grouting have been proposed and used in many government and private remediation sites. Additionally, though, long term (10""s of years) and short term (0 to 5 years) monitoring is required to address concerns of stakeholders as well as to satisfy regulatory requirements. Fundamental to many containment approaches is the need for a clear understanding of how fluids move through, accumulate in, and leave soils (or subsurface wastes and containment structures).
A need remains, therefore, for a reliable sensor system that can detect in situ the intrusion of moisture in soil wherein hazardous and/or radioactive waste is stored. The present invention aids in this understanding by providing a method and apparatus for monitoring fluid flow in situ.
It is an object of the present invention to provide a sensing system and method that detects moisture intrusion in soil using a simple apparatus operating in situ in soil containing wastes of concern.
It is another object of the present invention to provide a sensing system and method that infers the presence of moisture based on temperature differences detectable using a distributed temperature sensing element.
It is yet another object of the invention to rely on different thermal conductances of dry soil, slightly moist soil and saturated soil to detect presence and location of moisture in soil.
It is yet another object of the invention to utilize application of heat to soil in order to isolate where regions having different thermal conductances are located in a volume of soil being investigated (sometimes referred to as the xe2x80x9ctarget areaxe2x80x9d in this disclosure).
Yet another object is to provide a soil moisture sensing system that includes a linear element (such as a tube, cable or conduit) positioned in a volume of soil. Another related object is the provision of a distributed temperature sensing element associated with the linear element that permits identification of the regions having different thermal conductances mentioned above, especially when a heater is included, which, when actuated, heats the soil.
Yet another object of the invention is to provide a method for detecting moisture intrusion in soil which includes positioning in soil a linear element that comprises a distributed temperature sensing element, and further includes heating at least a portion of the soil, and measuring temperatures in the soil. Using the invention in this way, it is possible to ascertain whether regions are present in the soil suggesting different thermal conductances and, therefore, possibly the presence of moisture.
These and other objects are fulfilled and satisfied by the claimed invention which utilizes a linear element such as a cable, tube or conduit positioned in a serpentine fashion within a target area of soil. Optimal placement of the linear element assumes that it passes through a significant two-dimensional area and, preferably, also a significant three-dimensional volume within the target portion of the soil. The invention also includes using an optical fiber, that forms a component of a distributed temperature sensing system, and a heating element. The optical fiber serves as an optical conduit through which pulses of light pass, generating Raman scattered radiation within the target area. The scattered radiation is detected and analyzed using principles of optical time delay reflectometry (OTDR) to permit accurate and distributed temperature detection capability along the length of the fiber optic. The heating element which, in a primary embodiment disclosed here, is integrated with the cable or other linear element, is used to heat soil. Principles of the invention, though, would also be served using by an external heating apparatus. The heating causes regions of differing thermal conductance within the soil to become apparent and detectable by the distributed temperature detection system. According to the invention, those areas having relatively higher thermal conductance (possibly reflecting the presence of moisture) can thus be identified and located.
Additional advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.